458 AXEL A. OLSSON 



trapezoidal, inequivalve and inequilateral, slightly gaping behind, the right 

 valve larger than the left, the surface earthy, chalky or granulose, and 

 covered by a thin periostracum. Hinge edentulose, the hinge plate often 

 fissured. Ligament mostly internal, the tensilium small, placed above and 

 behind the resilium which is attached to a large, spoon-shaped chondrophore 

 in the hinge plate of both valves. Pallial sinus of moderate size. Surface 

 marked with growth lines only, or with concentric and sometimes oblique 

 undulations. 



Two genera are regional. 



I. Surface with growth lines only, often earthy. 



Genus Thracia 



II. Surface sculptured with concentric, sometimes more or less oblique, 

 wavelike plications, granulose. 



Genus Cyathodonta 



Genus THRACIA Blainville, 1824 

 Type species by subsequent designation, Blainville, 1827. Thracia cor- 

 buloidea Blainville. Mediterranean. 



In a final note, Blainville, 1827 (Man. Mai., vol. 2, p. 600) restricted 

 the name of Thracia to Division A, and thereby selecting T. corbuloidea as 

 the type species for the genus. Gray selected the same species as type in 1847. 



Shell oblong-ovate to trapezoidal in shape, inequivalve, the right valve 

 larger and more convex, the beaks often placed so close that they touch 

 and puncture each other. Texture of shell usually thin, concentrically striate, 

 and usually earthy, or with fine superficial granulations beneath a thin 

 periostracum. Valves with a small gap behind. Hinge edentulous. The ex- 

 ternal portion of the ligament small, the resilium larger and deeply immersed 

 into the hinge plate. A small lithodesma (often lost) may be present. 



Thracia colpoica Dall Plate 83, figures 7, 7a 



Thracia colpoica Dall, 1915, Proc. U.S. Nat. Museum, vol. 49, No. 2116, pp. 443, 444 

 Gulf of California. 



Shell small (length about 17 mm.), subovate, thin and papery, inequi- 

 lateral and strongly inequivalve, the right valve larger and more convex, 

 with high, full umbone and an oblique umbonal axis, the posterior side 

 shorter, depressed, bounded externally by an angle and carrying a sub- 

 median angle or rib within it. The left valve has a more ovate shape, de- 

 pressed to slightly convex. Surface somewhat earthy, the periostracum mostly 

 thin, heavier, and darker in color; on the posterior surface, markings mostly 

 concentric growth incremental. 



A specimen from Tumbez, measures: length 17 mm., height 16 mm., 

 diameter 8.7 mm. 



This species is common on the mud flat at the mouth of the Tumbez 

 River in Peru but the shell is so fragile that specimens are difficult to collect 

 or to preserve intact afterwards. 



Range — Gulf of California to northern Peru. Peru: Tumbez. 



Thracia anconensis, new species Plate 83, figures 4, 4a 



Left valve of medium size (length 34 mm.), slightly convex, sub- 



